Using in situ turbidity to estimate sediment loads in forested headwater streams : a top-down versus bottom-up approach Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ww72bd77h

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  • Suspended sediment and in situ turbidity data from two western Oregon streams, Oak Creek and South Fork Hinkle Creek, were used to estimate annual sediment loads for the 2006 water year (October 1, 2005 to September 30, 2006). Water samples and in situ turbidity observations were taken following the Turbidity Threshold Sampling (TTS) protocol. The annual hydrographs for Oak Creek and South Fork Hinkle Creek were divided into storms. This stratification resulted in storm-specific relationships between in situ turbidity and Suspended Sediment Concentration (SSC). The annual hydrograph for Oak Creek was separated into 15 storms. The annual hydrograph for South Fork Hinkle Creek was separated into 8 storms. In the relationship between SSC and in situ turbidity, especially for Oak Creek, there are counterintuitive values. Before statistical relationships between suspended sediment concentration and in situ turbidity could be developed, these counterintuitive and erroneous values had to be vetted. This was carried out with values of laboratory turbidity, hydrograph characteristics, and hysteresis loops. Observations of in situ turbidity considered erroneous were adjusted manually with the TTS-adjuster program. The estimates of sediment load determined with the TTS approach were defined as the true sediment load in the stream. The observations of in situ turbidity that were considered erroneous were also adjusted with a Turbidity-Threshold Macro (TTM), which automatically adjusted the turbidity record. The estimates of sediment load determined with the TTM approach are compared with the estimates of sediment load determined with the TTS approach to determine the efficacy of the TTM method. The objectives of this study were to determine the efficacy of an automated turbidity adjustment program compared with a manual turbidity adjuster, and to determine the efficacy of two in situ turbidity and SSC relationships to predict annual sediment loads. Relationships between SSC and in situ turbidity were made to estimate annual sediment load for Oak and South Fork Hinkle Creeks. The SSC vs. in situ turbidity relationships were made for storm-specific time periods and for the whole water year. Estimates of annual sediment load for Oak Creek were approximately 10 tonnes (2 percent) higher when the TTM-adjustment was made for annual and storm-specific relationships. Estimates of total annual sediment load for Oak Creek were approximately 100 tonnes (17 percent) higher when separate-storm relationships between SSC and in situ turbidity were used compared to an annual relationship. The estimates of annual sediment load at South Fork Hinkle Creek were much lower when the TTM-adjustment was compared to the TTS-adjusted record. When the annual relationship between in situ turbidity and SSC was used the estimate of annual sediment load for South Fork Hinkle Creek was 1,336 tonnes for TTM-adjusted turbidity data, compared to 1,526 tonnes for TTS-adjusted turbidity data. Estimates of total annual sediment load for South Fork Hinkle Creek were approximately 700-800 tonnes lower when the separate-storm relationships between SSC and in situ turbidity were used, compared to an annual relationship. The TTM method for adjusting in situ turbidity records was useful to remove spikes of in situ turbidity. In the case of Oak Creek, the TTM-adjuster worked satisfactorily. However, in the case of South Fork Hinkle Creek, the TTM-adjuster did not work as well. For neither stream was the TTM-adjuster able to recreate the record that resulted from the TTS-adjuster. A TTM-adjuster appears to be able to work well but it would be best used in conjunction with a final adjustment using the TTS-adjuster. Thus, a hybrid approach that uses the strengths of both approaches might be the best approach. The TTM-adjuster as presented in this thesis is not a finished product. No method was developed to calibrate a data set to a TTM threshold value. Thus, while a bottom-up, TTM-adjuster program to edit and adjust records of in situ turbidity appears to be viable, the details of the method are not perfected and it remains a work in progress. Both annual and storm specific relationships between in situ turbidity and SSC can be used to estimate sediment loads in streams. The record of success for these two methods depended on the stream. Oak Creek had a lot of samples (294) and the data was well-behaved. For that stream both approaches seemed to work well. However, South Fork Hinkle Creek had fewer samples (138) and the data was not as well behaved. It is probably best to use annual relationships when data is sparse or poorly behaved. Also, annual data alleviates the problem, to a degree, of extrapolating beyond the range of data that could be a problem for storm-specific relationships.
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